Problem: Which of these numbers is composite? ${2,\ 43,\ 61,\ 73,\ 77}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 2, 43, 61, and 73 each have only two factors. The factors of 77 are 1, 7, 11, and 77. Thus, 77 is the composite number.